Ionization pressure gauge



April 7, 1970 s. A. vEKsHlNsKY ET AL 3,505,554

IONIZATION PRESSURE GAUGE Filed Jan. 5, 1968 FIG-.l

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United States Patent Office 3,505,554 Patented Apr. 7, 1970 U.S.S.R.

Filed Jan. 5, 1968, Ser. No. 696,067 Int. Cl. H01j 41 00 U.S. Cl.313-157 4 Claims ABSTRACT OF THE DISCLOSURE An ionization pressure gagecomprises an anode cylinder mounted inside a casing with a cathodeextending along the axis of the anode cylinder. At one end of the anodecylinder is an ion collector electrode and at the other end of the anodecylinder is a shield, a magnetic lield being created to cause electronsemitted by the cathode to undergo spiral rotation as a cloud ofelectrons within the cathode-anode space. A control anode is disposedoutside of the cathode-anode space in close proximity to the cathode ata spacing which is chosen so that all electrons emitted in the area ofthe control anode fall thereon. The control anode may be of cylindricalshape and receive the cathode therewithin.

The present invention relates to apparatus for measuring low gaspressures, and more particularly to magnetron ionization pressure gaugeswith thermionic cathodes.

Ionization pressure gauges are known in which the system of electrodescomprises an anode cylinder, a thermionic cathode extending along theaxis of the anode cylinder, said cathode emitting electrons which forman electron cloud undergoing spiral rotary motion in the space betweenthe cathode and the anode cylinder under the action of a magnetic iield,a collector electrode and a shield, which latter two are disposed at therespective ends of the anode cylinder. The ion current in such pressuregauges is proportional to the gas pressure, provided the electronemission current I- from the cathode is constant, said electron emissioncurrent being equal to the electron current in the circuit including thecathode and the anode (the cathode-anode current) with the value of theapplied magnetic field H equalling zero. It is, therefore, necessary tomaintain constancy of the emission current I0- from the cathode, whenthe measurements of the gas pressure are carried out.

A disadvantage of the ionization pressure gauges of the known kind liesin the impossibility of control over the value of this emission currentI0- concurrently with the measurements of the gas pressure, since in thepresence of the magnetic field during the operational duty of the gauge,the anode current sharply decreases and its value is dependent upon thepressure, thus rendering impossible the evaluation of the emissioncurrent by the anode current value.

In another known kind of ionization pressure gauge, still anotherconstituent part is added to those mentioned above, this additional partbeing a control anode disposed adjacent to the cathode within thecathode-anode space, i.e. within the space occupied by the spirallyrotating cloud of electrons. This additional control anode enablesmeasuring and controlling the emission current, since the electroncurrent I1- fed to the control anode is proportional to the electronemission current I0* and is independent of the gas pressure within aconsiderably broad range of the gas pressure values.

However, the positioning of the control anode within the rotating cloudof electrons itself introduces a number of disadvantages, namely:

(l) The control anode current in the absence of the magnetic Iield isnot equal to the control anode current, when the magnetic field isapplied. Subsequently, it is quite diflicult to evaluate the truerelationship between the emission current IO- which is measured in theabsence of the magnetic field and the control anode current I1* which isused to judge and control the operation of the gauge.

Thus, with the magnetic field strength equalling zero, the currentarriving at the control anode is equal to If. When the value of theapplied magnetic field is above critical, a rotating cloud of electronsis formed about the cathode, and some of the electrons impinge on thecontrol anode, thus increasing the current applied to it, namely 11 I10.

(2) In case the potential at the control anode is varied by merefractions of a volt, as well as in case the position of the controlanode with respect to the rest of the electrodes is shifted butslightly, it immediately influences the operation of the wholeionization gauge, and, correspondingly, causes sharp variations in thecontrol anode current, as well as in the ion current and thecathodeanode current.

(3) The control anode current Il* is independent of the gas pressure(which fact makes it possible to evaluate and control the operationalduty of the ionization gauge), but this independence does not cover thewhole range of pressures at which the gauge may operate. Thus, it hasbeen found that with the emission current IU- equalling 1.1()9 A., thecontrol anode current I1* starts being dependent on the gas pressure P,when P equals 108 mm. of mercury. With the emission current being 1.10-8A., this dependence is detected at P=l07 mm. of mercury; and even withthe emission current as high as 1.106 A., the control anode current I1-starts varying with P=1()-6 mm. of mercury.

A further disadvantage inherent in the both above described kinds ofknown ionization pressure gauges is their comparatively complicatedstructure owing to the necessity of providing an exterior source of themagnetic ield.

It is an object of the present invention to provide an ionizationpressure gauge, in which the value of the electron current fed to thecontrol anode is independent of the value of the magnetic lield applied.

It is another object of the present invention to provide an ionizationpressure gauge in which the influence eX- erted by the control anode onthe operational characteristics or duty of the gauge is eliminated.

It is still another object of the present invention to provide anionization pressure gauge, in which the range of evaluated and controlvalues of the emission current should be extended over the whole rangeof the operational gas pressures measured by the gauge.

It is a further object of the present invention to provide an ionizationpressure gauge of simplified structure, owing to the elimination of theexternal source of magnetic field.

With these and other objects in View, the ionization pressure gaugeembodying the present invention comprises a control anode positionedexteriorly of the cathode-anode space with its rotating cloud ofelectrons, said control anode being located at close proximity to thecathode, the spacing between said control anode and said cathode beingso chosen that all electrons emitted by said cathode within the areathereof covered by said control anode fall onto the latter.

In a preferred embodiment of the present invention, the control anode ispositioned outside the cathode-anode space where the rotating cloud ofelectrons is formed, on the metal shield side.

Also in a preferred embodiment of the present invention the controlanode acquires the form of a cylinder spacingly receiving the cathodethereinside and having the radius which is less than the distance of thespreading of electrons from the cathode under the combined action of theelectric and magnetic fields.

In order to create a spirally rotating cloud of electrons in thecathode-anode space of the guage, the anode cylinder may be made of amagnetic material.

According to these novel features of the present invention, theinfluence of the magnetic field strength H on the electron current I1-applied to the control anode is eliminated; also eliminated is theinfiuence of the control anode itself on the operational duty of thegauge, the range of the evaluated and controlled values of the emissioncurrent is extended over the whole range of pressures measured by thegauge, and the structure of the gauge is simplified.

Other objects and advantages of the present invention will be betterunderstood from the following detailed description of an embodimentthereof, with due reference to the accompanying drawing, in which:

FIG. 1 is a schematic sectional vie-w of the ionization pressure gaugeemboding the present invention;

FIG. 2 shows the same pressure gauge, as in FIG. 1, but with the anodecylinder made of a magnetic material;

FIG. 3 is a graph showing the dependence between the control anodecurrent Iland the emission current If;

FIG. 4 is a graph showing the relationship between the control anodecurrent I1- and the magnetic field strength H with a constant value ofthe emission current If; and

FIG. 5 is a graph showing the dependence between the current If appliedto the control anode and pressure with the value of the emission currentI being constant.

In the drawing, the system of the electrodes of an ionization pressuregauge embodying the present invention comprises an anode cylinder 1(FIG. l), an ion collector electrode 2, a shield 3, a control anode 4and a cathode 5. The collector electrode 2 and the shield 3 are shapedas discs disposed at the opposite ends of the anode cylinder 1. Thecontrol anode is shaped as a cylinder arranged coaxially with the anodecylinder 1, -with the shield 3 located intermediate the anode cylinder 1and the control anode 4. The cathode extends along the axis of the anodecylinder 1 and the control anode 4 through openings in the collectorelectrode 2 and the shield 3, with a spring 6 provided for tensioningthe cathode. A holder 7 of the cathode S provided on the collectorelectrode 2 side of the gauge projects by several millimetres into thecathode-anode space, for the hot thermionic portion of the cathode 5 tolie in the area where a potential barrier exists, which prevents thetravel of positive ions formed at the cathode 5 to the collectorelectrode 2.

In a modification of the ionization pressure gauge embodying the presentinvention, the shield 3 may have no opening. In this case the shield 3separates the cathode 5 into two portions and is maintained under thepotential of the cathode.

In order to be connected to a source of vacuum, the whole system of theelectrodes is mounted inside a glass container 8. Alternatively, thesystem of the electrodes may be mounted inside a metal container orarranged in an exposed state on a suitable flange with the use ofceramic insulation members.

The magnetic field needed for the operation of the pressure gauge iscreated by a hollow cylinder-shaped permanent magnet 9. Alternatively,the magnetic field can bc created by an electromagnet coil or asolenoid.

Shown in FIG. 2 is an ionization pressure gauge in which the anodecylinder 1' itself is made of a material having magnetic properties andis a cylinder-shaped hollow .permanent magnet providing the magneticfield for the operation of the gauge. This feature makes it possible todo without a separate exterior magnet and thus to simplify the structureof the pressure gauge.

Below is an example of an operational characteristics of the pressuregauge shown in FIG. 1:

Potential difference between the anode cylinder and cathode 5=+400 v.;

Potential difference between the control anode 4 and cathode 524-400 v.

Potential difference between the cathode 5 and ground Potentialdifference between the collector electrode 2 and cathode 5:-200- v.;

Potential difference between the shield 3 and cathode Electron emissioncurrent I0*=l.l07 A.;

Magnetic field strength H=450 oersteds.

-For the electron current Ilthrough the control anode 4 to stay at aconstant value with the magnetic field strength H lbeing increased fromzero to an operational value, it is necessary that the minimal spacingbetween the cathode 5 and the control anode 4 be less than the distanceof the spreading of electrons from the cathode 5 under the combinedaction of the electric and magnetic fields. For a cylinder-shapedcontrol anode the above requirement may be exposed as follows:

waxy/if where r is the radius of the control anode 4 in mm.;

H is the strength of the magnetic field in oersteds;

V is the potential difference between the control anode 4 and thecathode S in volts.

Accordingly, the radius r of the control anode 4 should not exceed 3 mm.for the operational characteristics of the pressure gauge, as givenabove.

The shield 3 is located intermediate the anode cylinder 1 and thecontrol anode 4 in order to eliminate the interaction and mutualinfluence of their respective electric fields. Due to negative potentialof the shield 3 with respect to the cathode 5, migration of electronsfrom the area of the control anode 4 to that of the anode cylinder l isprevented and vice versa. In this manner the influence exerted by thecontrol anode 4 on the operational characteristics of the pressure gaugeis eliminated, and the electron current value I1- through the controlanode 4 can ybe maintained constant.

With the voltage supplied to the electrodes being maintained at constantvalues, and the applied magnetic field strength equalling zero, theelectron current in the anode cylinder 1, which is equal to the emissioncurrent If, and the electron current If in the control anode 4 aredefined by the temperature of the cathode 5, and a given value of I0-brings about a corresponding value of I1- for any value of the pressureP below 1.10*4 mm. of mercury. This is illustrated in FIG. 3, where theabscissa axis corresponds to the values of the emission current I0- inamperes, and the ordinate axis shows the values of the control anodecurrent Ilin ampers.

In the absence of the magnetic field, i.e. with the magnet 9 removed,the emission current IO- is adjusted to an operational value, and thecorresponding control anode current I1- is measured. When the magneticfield is now applied, whose strength is above the critical point,electrons emitted by the cathode 5 start rotating thereabout, forming aspirally rotating cloud of electrons. Under these conditions only asmall fraction of the total number of the electrons reach the anode .1,whereby the electron current fed to the anode 1 is sharply reduced. Themolecules of a gas, present in the area of the rotating cloud ofelectrons are ionized, and the resulting ions migrate to the collectorelectrode 2. The value of the ionic current applied to the collectorelectrode 2 is a measure of Igas pressure. .The control anode currentI1- is not changed by the application of the magnetic field, as isillustrated in the graph shown in FIG. 4, where the abscissa indicatesthe magnetic field strength H in oersteds, and the ordinate indicatesthe corresponding values of the current I1- in amperes in control anode4.

Strict correspondence of the current I1- in control anode 4 to theemission current IO- is established over a wide range of the pressuresmeasured, which is illustrated in FIG. 5 where the abscissa shows gaspressures P in mm. of mercury, while the ordinate indicates the emissioncurrent I0- and the control anode current Il in amperes.

By evaluating and stabilizing the current Ilin control anode 4 in theprocess of taking measurements, the operational characteristics of thepressure gauge may be thus evaluated and stabilized.

When operating an ionization pressure gauge shown in FIG. 2, in whichthe anode 1 acts at the same time as a permanent magnet, it isimpossible to measure the emission current I0, and the operationalcharacteristic of the pressure gauge is set according to the values ofthe current Il in control anode 4 which is evaluated and stabilized inthe process of taking pressure measurements.

From the above description it has been made clear that in an ionizationpressure gauge with a control anode, embodying the present invention,the electron current through the control anode is virtually unaiected bythe strength of the magnetic iield applied, the influence of the controlanode on the operational characteristics of the gauge is eliminated, andthe range of controlled and sta-bilized values of the control anodecurrent, and, subsequently, of the emission current is extended over thewhole range of the operation of the gaufie, with the structure of thegauge itself considerably simplified.

Although the present invention has been described in connection with apreferred embodiment thereof, it should be understood that variouschanges may be introduced and various modifications may be made withoutdeparting from the spirit and scope of the invention, as those skilledin the art will be sure to appreciate.

What is claimed is:

1. An ionization pressure gauge comprising a casing, an anode cylindermounted inside said casing, a cathode extending along the axis of saidanode cylinder and adapted to emit electrons, an ion collector electrodelocated at one end of said anode cylinder, a shield located at theopposite end of said anode cylinder, means for creating a magnetic iieldunder the action of which the electrons emitted by said cathode form aspirally rotating cloud of electrons within the cathode-anode spacedened by said anode cylinder, said cathode, said ion collector electrodeand said shield; a control anode disposed exteriorly of saidcathode-anode space with the rotating cloud of electrons, said controlanode being arranged in close proximity to said cathode, the spacingbetween said controlanode and said cathode being so chosen thatsubstantially all electrons emitted by said cathode in the area coveredby said control anode fall onto the latter.

2.. An ionization pressure gauge, as set forth in claim 1, in which saidcontrol anode is disposed exteriorly of said cathode-anode space withthe rotating cloud of electrons, on the same side of said anodecylinder, as said shield.

3. An ionization pressure gauge, as set forth in claim 1, in which saidcontrol anode is shaped as a cylinder receiving said cathodethereinside, said cylinder having a radius which is smaller than thedistance of the spreading of the electrons from said cathode under thecombined action of the electric and magnetic elds present.

4. An ionization pressure gauge, as set forth in claim 1, in which saidanode cylinder is made of magnetic material adapted to create a magneticfield, under the action of which a rotating cloud of electrons can beformed within said cathode-anode space.

References Cited UNITED STATES PATENTS 3,239,715 3/1966 Laierty 313-7 XRAYMOND F. HOSSFELD, Primary Examiner U.S. Cl. X.R.

